Air conditioning system for an automobile

ABSTRACT

In an air conditioning system for a motor vehicle with a compressor that comprises at least one cylinder with movable piston ( 2 ), to improve the sealing action between the skirt of the piston ( 2 ) and the cylinder wall ( 1 ) it is proposed to dispose on the skirt of the piston ( 2 ) a labyrinth seal and/or to dispose a ring ( 10 ) made of a flexible sealing material in a slot ( 8 ) within which a piston ring ( 9 ) is mounted.

DESCRIPTION

The invention relates to a vehicle air conditioning system with acompressor according to the precharacterizing clause of claim 1.

Air conditioning systems of this sort have previously been operated withcoolants such as R12 or R134a. The operation of such a system requiresthe compressor to be capable of producing a pressure difference of ca.10 to 20 bar between suction pressure and counterpressure. This can beachieved with compressor pistons comprising no or at most one pistonring.

Organic halogen coolants have recently been publicly criticised becausewhen released into the atmosphere, they can have an ozone-decomposingaction and aggravate the greenhouse effect. Therefore considerableefforts have been made to develop air conditioners that operate withcoolants that pose no risk to ozone, i.e. are neutral with respect tothe greenhouse effect. Here the problem arises that substancespotentially suitable as coolants, inasmuch as they meet theserequirements, demand considerably higher pressure differences. Suchdifferences are not readily achievable with the conventionalconstructions of air-conditioner compressors. When an attempt is made toemploy such coolants in a compressor with a conventionally constructedpiston, without piston ring or with a single piston ring, the efficacyof such a compressor is seriously impaired by leakage losses owing toflow of the coolant along the skirt of the piston, and there is a riskthat these will make the system entirely nonfunctional. Therefore in thecase of air conditioning systems employing coolants that must be greatlycompressed, cylinders have been used in which the pistons bear as manyas four rings. A compressor for a vehicle that comprises one cylinder isdisclosed, for example, in the U.S. Pat. No. 4,594,055. In order toimprove the seal between piston and cylinder, the piston comprises onits skirt two grooves that run in the circumferential direction, withpiston rings positioned in each. In comparison to a piston with only onepiston ring, this piston is more difficult to tilt within the cylinder,as a result of which wear and tear on the inner cylinder wall isreduced. The seal between piston and cylinder is also improved. However,this increases the effort and expense of constructing the compressor.Furthermore, the friction of the piston against the inner walls of thecylinder is considerably increased, so that operation of the compressorrequires extra work that contributes nothing to the cooling action. Onthe contrary, the frictional heat generated between piston and cylinderis largely absorbed by the coolant, which is absolutely undesirable.

These disadvantages are not present in the air conditioning unit withthe features given in Claim 1. Additional advantageous embodiments ofthe invention will be evident from the dependent claims.

Advantages of the Invention

By means of the invention an automobile air conditioning system iscreated that, by limiting leakage losses in the compressor, enablesoperation with coolants that must be greatly compressed, while avoidingan appreciable increase in friction as compared to air conditionersdesigned for smaller pressure differences.

Such an air conditioner is particularly well suited for operation withCO₂ as coolant in the so-called transcritical process, or with othercoolants that require large differences between suction pressure andcounterpressure, ranging from over 20 bar up to 100 bar.

In a first air conditioning system in accordance with the invention, thepiston is provided with a labyrinth seal at its skirt surface. Such alabyrinth seal, which preferably comprises a plurality of groovesoriented in the circumferential direction of the skirt surface, is alsosuitable for reducing leakage losses particularly in the case of highpressures. The reason is that it is just when the pressure of thecoolant contained in the cylinder is highest, a time when particularlylarge leakage losses are to be expected, that the viscosity of thecoolant is no longer negligible because of the severe compression, andas a result the turbulence of the coolant escaping from the cylinderbetween its inner wall and the piston skirt, with the grooves of thelabyrinth seal, presents an effective resistance to flow.

In a second embodiment of the air conditioning system in accordance withthe invention, in which the piston is conventionally constructed insofaras it comprises at least one slot within which a piston ring is mounted,it is provided that there is additionally disposed in the slot a ringmade of a flexible sealing material. The function of this ring is toconstrict or completely block the path of the coolant, which in the caseof a piston with piston ring runs through the slot substantially alongits outward-facing surfaces and along the inner circumferential surfaceof the piston ring.

For this purpose the ring is disposed substantially between the pistonring and a side wall of the slot at which the pressure is relativelylow, so that the piston ring is pressed by the coolant against theflexible ring, as a result of which a sealing action of the latter isachieved. The flexible ring itself advantageously does not extendoutward beyond the surface of the piston skirt, so that it does not comeinto contact with the inner wall of the cylinder; therefore it is notsubject to frictional wear and tear and also does not increase the forceneeded to overcome friction in order to drive the piston.

It is further preferred that the piston ring be held within the slotwith axial play. This can enable the coolant to penetrate from thehigh-pressure side into a space between the inner surface of the pistonring and the bottom of the slot, and thus to press the piston ringuniformly, over its entire circumference, against the inner wall of thecylinder. It can be sufficient in this regard for this play to begenerated only in a high-pressure phase of the working cycle of thecompressor, by axial compression of the ring that is made of flexiblesealing material.

A space can also be provided between the bottom of the slot and theinner surface of the ring made of flexible sealing material, so that thelatter, just like the piston ring, is pressed radially outward bycoolant that penetrates into the space and thus reinforces thecorresponding extension movement of the piston ring.

Additional characteristics and advantaces will be apparent from thefollowing description of exemplary embodiments.

FIGURES

FIG. 1 shows a section of a compressor cylinder for an air conditioningsystem according to a first embodiment of the invention; and

FIG. 2 shows a section of a cylinder according to a second embodiment ofthe invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The drawing in FIG. 1 shows only part of a cylinder for a compressor inan air conditioning system in accordance with the invention. The otherelements of such a system are known and need not be treated here indetail.

FIG. 1 shows a section of a cylinder wall 1 and of a piston 2 that canbe shifted within the cylinder along its central axis, indicated by adot-dash line 3. The piston 2 comprises a skirt that is closely apposedto the cylinder wall 1 (the gap 4 between the two is shown larger thanto scale in the figure, for the sake of clarity). On the skirt of thepiston 2 is disposed a plurality of slots 5 oriented in thecircumferential direction; a section of this region is shown enlarged onthe right side of the figure. In this circle it can be seen that theslots 5 have a length in the direction of the axis 3 as well as a radialdepth, which corresponds approximately to the width of the gap 4 betweenthe piston and the cylinder wall 1. Coolant that escapes from thecylinder in the direction of the arrow 6, during the high-pressure phaseof the cylinder, becomes turbulent at each individual slot 5 asindicated by the arrows 7, so that no uniform, laminar leakage flow candevelop in the gap 4. Thus a considerable reduction of the leak currentis achieved, with no narrowing of the gap 4 in comparison toconventional piston-cylinder constructions. The friction betweencylinder wall and piston is therefore no higher than in conventionalconstructions.

To reinforce the action of the labyrinth seal, piston rings canadditionally be provided at the piston.

FIG. 2 shows in section a piston-ring seal in accordance with theinvention. This can be used by itself or in combination with thelabyrinth seal described above.

In FIG. 2 a section of a cylinder wall 1 can be seen as well as asection of a piston 2 that can be moved along this cylinder wall. A slot8 extends in the circumferential direction over the skirt of the pistonas do the slots in FIG. 1, but has a considerably greater length anddepth than do the latter. In the slot 8 are disposed a piston ring 9 aswell as a ring 10 made of a flexible sealing material. The outercircumferential surface of the piston ring 9 is apposed to the cylinderwall 1; between the inner circumferential surface of the ring 9 and thebottom 12 of the slot 8 there is a space 13. The space 13 communicateswith the pressure chamber of the cylinder (not shown) by way of a gap14, which extends along the end face of the piston ring 9 on thehigh-pressure side, and the gap 4 between cylinder wall 1 and skirt ofthe piston 2.

Between the end face 16 of the ring on the low-pressure side and theside wall of the slot 8 adjacent thereto is disposed the ring 10 ofsealing material. This ring 10 is advantageously fixed firmly to the endface 16. In an arrangement alternative to that shown in the figure, itcan extend up to the high-pressure end face 15 of the piston ring, inthe form of a multi-sided coating. The space 13 also extends between theinner circumferential surface of the ring 10 and the bottom 12 of theslot 8. The outer circumferential surface of the ring 10 is set slightlyback, towards the axis, so that it is behind the skirt surface of thepiston 2.

When the end face 15 is exposed to a high pressure during thehigh-pressure phase of the cylinder, it can move back in the axialdirection, i.e. downward in the figure, with simultaneous compression ofthe ring 10. As a result the gap 14 becomes wider and coolant penetratesinto the space 13. The pressure of the coolant on the innercircumferential surface 11 of the piston ring 9 presses the latteragainst the cylinder wall 1 and thus ensures an effective seal betweenthese two surfaces. At the same time it presses the ring 10 radiallyoutward, so that when necessary this ring expands together with thepiston ring 9 and thus does not impede its outward movement. The axiallycompressed ring 10 simultaneously creates an effective seal between thelow-pressure-sided end face 16 of the piston ring and the adjacent sidewall of the slot 8.

The dimensioning of the ring 10 ensures that even under axialcompression it will not come into contact with the cylinder wall 1, sothat it is not exposed to frictional wear and tear.

The frictional force that must be employed to move a piston sealed inaccordance with the invention does not differ substantially from thatneeded for a piston with conventional piston rings. Nevertheless, theconsiderably improved sealing action allows the piston in accordancewith the invention to be used for applications with considerablyelevated pressures, such as occur in particular in an air conditioningsystem that employs a high-pressure coolant such as, for example, CO₂.

What is claimed is:
 1. Air conditioning system for a motor vehicle, witha compressor that comprises at least one cylinder having a movablepiston (2), wherein the piston comprises in its skirt at least one slot(8) in which a piston ring (9) is mounted, characterized in that in theslot (8) a ring (10) made of a flexible sealing material is disposed. 2.Air conditioning system according to claim 1, characterized in that thering (10) is disposed substantially between a side wall of the slot (8)on its low-pressure side and the piston ring (9).
 3. Air conditioningsystem according to claim 1, characterized in that the ring (10) doesnot project beyond the skirt surface of the piston (2).
 4. Airconditioning system according to claim 1, characterized in that thepiston ring (9) is held in the slot (8) with axial play.
 5. Airconditioning system according to claim 1, characterized in that a space(13) is provided between the bottom (12) of the slot (8) and an innersurface of the ring (10).
 6. Air conditioning system according to claim1, characterized in that carbon dioxide is provided as the coolant. 7.Air conditioning system according to claim 1, characterized in that theflexible ring (10) is attached to the piston ring (9) as a fixed coatingon one or several sides.
 8. Air conditioning system according to claim1, characterized in that it generates pressure differences of over 50bar between suction pressure and counterpressure.